In the book, Proudhon most famously declared that “property is theft”. Proudhon believed that the common conception of property conflated two distinct components which, once identified, demonstrated the difference between property used to further tyranny and property used to protect liberty. He argued that the result of an individual's labor which is currently occupied or used is a legitimate form of property. Thus, he opposed unused land being regarded as property, believing that land can only be rightfully possessed by use or occupation (which he called "possession"). As an extension of his belief that legitimate property (possession) was the result of labor and occupation, he argued against such institutions as interest on loans and rent.
Proudhon contrasted the supposed right of property with the rights (which he considered valid) of liberty, equality, and security, saying: "The liberty and security of the rich do not suffer from the liberty and security of the poor; far from that, they mutually strengthen and sustain each other. The rich man’s right of property, on the contrary, has to be continually defended against the poor man’s desire for property." He further argued that the right of property contradicted these other rights: "Then if we are associated for the sake of liberty, equality, and security, we are not associated for the sake of property; then if property is a natural right, this natural right is not social, but anti-social. Property and society are utterly irreconcilable institutions."
Though Proudhon rejects the right of property per se, he also argues that the state of possession as it is (or was) could not be justified even by supposing this right. Here he feigns to bring a legal claim against society, in a style mocking legal rhetoric:
Proudhon claims that his treatise "shall prove beyond a doubt that property, to be just and possible, must necessarily have equality for its condition." He used the term mutuellisme (mutualism) to describe his vision of an economy in which individuals and democratic workers associations could trade their produce on the market under the constraint of equality.
Some contemporary anarchists use the terms personal property (or possessive property) and private property to signify the distinctions Proudhon put forth in regard to ownership of the produce of labor and ownership of land. In this sense, private property would refer to claimed ownership of unused land or goods, and personal property would refer to produce of labor currently in use. This differentiation is an important component in anarchist critique of capitalism.